Implement ability to limit module documentation building (#24576)

* Implement ability to limit module documentation building:

- Added new option to plugin_formatter.py to support passing-in a list of
  modules for which the documentation should be built.
- Updated docuemtnation Makefile to allow specifying list of modules via
  environment variables (defaulting to all modules).
- Update instructions for building documentation and module development to
  include commands and description of limiting module documentation builds.

* Updated implementation for limiting module documentation building:

- Pass list of modules (or None) to list_modules function instead of string.
- Move conversion of module list to argument parsing code.
- No special keywords. Default ("") means build all modules. For no modules just
  specify non-existing module name.
- Updated documentation to reflect the changes.

* Updated implementation for limiting module documentation building:

- Use better default value, and don't treat "" as special case.
- Conditionally invoke different variants of command in Makefile instead of
  using special value "".

* Minor edits

Wording tweak

docs/docsite/README.md

@@ -14,6 +14,12 @@ such as link references, you may install sphinx and build the documentation by r
 To include module documentation you'll need to run `make webdocs` at the top level of the repository.  The generated
 html files are in docsite/htmlout/.
 
+To limit module documentation building to a specific module, run `MODULES=NAME
+make webdocs` instead. This should make testing module documentation syntax much
+faster. Instead of a single module, you can also specify a comma-separated list
+of modules. In order to skip building documentation for all modules, specify
+non-existing module name, for example `MODULES=none make webdocs`.
+
 If you do not want to learn the reStructuredText format, you can also [file issues] about
 documentation problems on the Ansible GitHub project.